The present invention relates to methods and compositions for consolidating formation fines, reducing proppant flow-back, and consolidating relatively unconsolidated portions in a subterranean formation (referred to herein collectively as “particulate migration”). More particularly, the present invention relates to the use of novel delayed tackifying compositions for reducing particulate migration in subterranean formations. While suitable for any subterranean application (such as those involving carbonates, sandstones, shales, coals, etc.), the compositions and methods of the present invention may be especially useful in coal bed methane (“CBM”) subterranean applications.
Hydraulic fracturing is a process commonly used to increase the flow of desirable fluids from a portion of a subterranean formation. Traditional hydraulic fracturing operations comprise placing a fracturing fluid into a portion of a subterranean formation at a rate and pressure such that fractures are formed or enhanced into the portion of the subterranean zone. The fractures tend to propagate as vertical and/or horizontal cracks located radially outward from the well bore. In such treatments, once the hydraulic pressure is released, the fractures formed will tend to close back onto themselves. To prevent this, oftentimes particulate materials, known as proppant, are placed in the fractures by transporting them in the fracturing fluid during at least a portion of the fracturing operation. The particulates are carried into created or natural fractures and deposited therein such that when the hydraulic pressure is released the particulates act to prevent the fracture from fully closing, and thus, aid in forming conductive channels through which produced fluids may flow into the well bore. The term “propped fracture” as used herein refers to a fracture (natural or otherwise) in a portion of subterranean formation that contains some proppant particulates. The term “proppant pack” refers to a collection of a mass of proppant particulates within a fracture. Without the particulate materials, the fractures tend to close and reduce permeability gained by the fracturing operation.
Hydrocarbon wells are often located in subterranean zones that contain unconsolidated particulates (e.g., proppant and formation fines) that may migrate within the subterranean formation with the oil, gas, water, and/or other desirable fluids produced by a well. The presence of these unconsolidated particulates in produced fluids is disadvantageous and undesirable in that the particulates may abrade pumping and other producing equipment and reduce the fluid production capabilities of producing zones. The particulates also may impact negatively the permeability of the formation. Unconsolidated subterranean formations include those that contain portions that contain loose particulates (e.g., proppant and formation fines) and those wherein the bonded particulates have insufficient bond strength to withstand the forces produced by the production of fluids through the zones.
Controlling particulate migration in coal bed methane applications may be particularly important. “Coal bed methane” (“CBM”) is the name usually given to methane found within coal seams. The amount of methane produced from a coal bed depends at least in part on the degree of permeability that is controlled by the amount of fracturing or cleats within the coal bed. CBM formations tend to have a naturally low permeability. These formations also are typically associated with low temperatures (e.g., less than 200° F.) and low reservoir pressures (e.g., less than 1000 psi bottom hole pressure). High capillary forces within the pore spaces tend to hold treatment fluids therein. Coal fines can be generated from the coal. These coal fines, or other particulates, can migrate and plug or partially plug the perforations, cleats, fractures, proppant, and/or producing zones.
One traditional method of controlling unconsolidated particulates in zones of a subterranean formation involves placing a filtration bed containing gravel particulates near the well bore that neighbors the zone of interest. The filtration bed acts as a sort of physical barrier to the transport of unconsolidated particulates to the well bore that could be produced with the produced fluids. Typically, such so-called “gravel packing operations” involve the pumping and placement of a quantity of desired particulates into the unconsolidated formation in an area adjacent the well bore. One common type of gravel packing operation involves placing a sand control screen in the well bore and packing the annulus between the screen and the well bore with gravel of a specific size designed to prevent the passage of formation sand. The sand control screen is generally a filter assembly used to retain the gravel placed during gravel pack operation. A wide range of sizes and screen configurations are available to suit the characteristics of the gravel pack sand used. Similarly, a wide range of sizes of gravel is available to suit the characteristics of the unconsolidated particulates. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow. When installing the gravel pack, the gravel is carried to the annulus in the form of a slurry by mixing the gravel with a fluid, often known as a “gravel pack fluid.” Sometimes gravel pack fluids are viscosified with suitable gelling agents. Once the gravel is placed in the well bore, the viscosity of the fluid is reduced, and it is returned to the surface. In some gravel packing operations, commonly known as “high rate water packing operations,” the fluid has a lower viscosity and yet the gravel is transported because the treatment occurs at a high velocity. Gravel packs act, inter alia, to stabilize the formation while causing minimal impairment to well productivity. The gravel, inter alia, acts to prevent the particulates from occluding the screen or migrating with the produced fluids, and the screen, inter alia, acts to prevent the gravel from entering the production tubing. Such packs may be time consuming and expensive to install.
Another method used to control particulates in unconsolidated formations involves consolidating unconsolidated portions of subterranean producing zones into relatively stable permeable masses by applying a resin followed by a spacer fluid and then a catalyst. Such methods may be problematic when, for example, an insufficient amount of spacer fluid is used between the application of the resin and the application of the external catalyst. In that case, the resin may come into contact with the external catalyst in the well bore itself rather than in the unconsolidated subterranean producing zone. Furthermore, there is uncertainty as to whether there is adequate contact between the resin and the catalyst. Additionally, when resin is contacted with an external catalyst an exothermic reaction occurs that may result in rapid polymerization, potentially damaging the formation by plugging the pore channels. Uniform placement of curable resin into the formations having long intervals is most desirable. However, formations often comprise a wide range of permeabilities even within a single reservoir located along a well bore. As a result, completions involving resin consolidation, with conventional diversion techniques, have been applied in intervals of less than 50 feet, and more ideally, less than 30 feet. Also, using resins to consolidate long or large unconsolidated zones may not be practical due, at least in part, to the high cost of most suitable resins.
Another similar method involves applying a non-aqueous tackifying composition to the unconsolidated particulates in an effort to reduce the migration of particulates within the zone. Whereas a curable resin composition produces a hard mass, the use of a non-aqueous tackifying composition produces a more malleable consolidated mass.
Another alternative is an aqueous tackifying composition. Aqueous tackifying compositions, however, have their own problems including, but not limited to, the fact that they require external activators and surfactants for optimum performance.
A new technique that could be useful is a fracturing treatment wherein the fracturing fluid comprises a suitable consolidation agent that reacts in such a way as to delayingly consolidate particulates within the formation to prevent particulate migration. This has heretofore not been accomplished, inter alia, because of the limitations associated with conventional acids and acid anhydride activators.